The general objective is to increase our understanding of the biology of carcinogenesis. Carcinogenesis is interpreted to include the whole series of events and processes that result in a fully developed, metastasizing neoplasm. A major portion of the work is therefore concerned with the interactions of a neoplastically transformed cell or clone with the host. In particular, emphasis will be given to the investigation of a newly discovered property of the immune response, namely specific tumor immunological stimulation. For this purpose both in vivo transplantation studies and in vitro cytotoxicity tests will be used. The "nude" mouse in the germ-free state will be a principal tool. Additionally, the mechanisms of the immunological stimulation will be investigated and their role in the natural biology of cancer determined. It is hoped that these studies will give insight into ways in which immunological stimulation can be prevented and immunological surveillance made more effective.